This invention relates broadly to the recovery of reusable materials from refuse, and particularly to the recovery of broken glass from particles of metal, ceramic and other heat resistant materials.
Much attention has been directed lately to the recovery of various materials from municipal refuse so that they may be recycled and reused. Glass is one such material, since glass cullet, i.e., broken glass, is an important ingredient in glass manufacture. Cullet, added to the basic ingredients of glass (sand and soda ash), physically aids in the reaction by conducting heat and thus reducing overall energy used in the melting process by up to 20%, and if there is not sufficient cullet available from in-house rejects, glass manufactures often recycle part of their production. Thus glass recovered from refuse has a ready use as cullet, provided it is free of contaminant metal, ceramic, stones and the like.
In order to use glass recovered from refuse as cullet, it should be free of unmeltable material and metals. Contaminants produce flaws in glassware and plug the spinnerettes used in glass fiber manufacture. The major contaminants in glass recovered from refuse are aluminum, iron, high melting point stones, and refractories, such as brick, porcelain and chinaware. The procelain insulators on spark plugs can be a particularly troublesome contaminant. Generally three methods have been considered for mechanically separating contaminants from the glass recovered from refuse, namely optical sorting, froth flotation, and dense media flotation.
Optical sorting normally involves reducing the particle size of the mixture containing the glass to less than one inch, screening to remove fines, separating out the organic materials, removing any metal, and finally using optical sorters to remove opaque ceramic materials and sort the glass by color. An example of such a process is set forth in U.S. Pat. No. 3,802,558 to H. S. Rhys. Such a process can produce flint, green and amber colored cullet, but it has a practical disadvantage in that it can best handle particles of a limited size range, generally one-quarter to three-quarter inch in diameter, because of the fact that the particles must be passed individually and serially past the optical sensors, thereby limiting productive capacity.
In the froth flotation method, the refuse is dry shredded; the organics are floated off using a dense media or rising current method; the metals are removed; the glass, stones and ceramics mixture is ground to -20 mesh; and finally the glass is froth floated off using chemical surfactants while the nonglass sinks. The froth flotation method suffers from a number of disadvantages including the fact that the -20 mesh material tends to blanket and insulate the melt in the glass furnace and to blow away causing air pollution. Further, grease and other chemical contaminants interfere with the effectiveness of the flotation process.
In the dense media method, the refuse is reduced to a mixture of only glass, stones and ceramics, and a dense media is then used to float the glass, while the contaminants sink. The dense media method suffers from the disadvantage that the specific gravity of glass is close to that of the contaminants, resulting in poor separation and reduced glass yield.
A need therefore exists for an improved method for separating glass from heat resistant contaminants which is simple, efficient and inexpensive.